Photographic technology has enjoyed an ever increasing growth since its inception more that 150 years ago. What began as a curiosity requiring long exposure times to produce low quality image reproductions has now advanced to become a large and diverse technology having widespread utility for both commercial and recreational purposes.
Photography has, until recently, been a process based upon chemical reactions, typically of photosensitive salts of precious metals such as silver and platinum. While such processes have been optimized to provide high sensitivity, good resolution and reliable performance, they suffer from several shortcomings. Chemically based photographic systems consume relatively large amounts of both precious metals and specially synthesized organic chemicals; consequently, such chemical processes tend to be fairly expensive. Furthermore, chemical processes require fairly strict control of time and temperature conditions in order to produce uniformly reliable results. Additionally, chemical based photographic systems require the storage and deployment of relatively large amounts of photographic film within a camera, and necessitate, in most instances, complex processing equipment.
Due to, inter alia, the foregoing limitations of chemically-based photographic systems, the photographic industry has explored the possibility of adapting presently emerging electronic imaging technologies. Image scanners are enjoying growing utility in a variety of products and for a diversity of applications such as, without limitation, television cameras, input of alpha numeric data to computers and machine vision systems. Optical scanning systems typically include one or more photosensor arrays, each array including a plurality of photoresponsive elements. For purposes of describing the present invention, the terms "photosensitive element" and "photoresponsive element" shall be interchangeably employed and broadly applied to include any element capable of producing a detectable electrically digitized signal in response to the absorption of illumination incident thereupon. By way of example, such detectable signals may be provided by a detectable change in voltage, current, resistivity, capacitance or the like.
Electronic arrays of photosensitive elements are capable of providing a signal corresponding to a pattern of information projected thereupon; and consequently, such arrays may be utilized to partially replace conventional photographic film by providing an electronic imaging system free of the shortcomings inherent in chemically based photographic systems.
Charge Coupled Devices (CCDs) represent one type of photosensor array which has heretofore been employed in electronic photographic applications. CCDs are solid state devices, typically formed from single crystal silicon and which include therein an array of photoresponsive elements. CCDs are highly photosensitive and are capable of providing high resolution images. However, CCDs are relatively small in size; the typical CCD array being a two dimensional matrix approximately one centimeter square. The largest CCDs currently produced are one dimensional arrays no greater than approximately 3 to 4 inches in length. Obviously, these size constraints impose restrictions on the utility of CCDs for electronic photographic applications. It is to be noted that, while CCDs generally provide a high degree of resolution, in order to have commercial impact and to be of practical utility for photographic applications, an optical reduction system must be employed. Since such optical systems project a reduced size image of the object being photographed onto the surface of the CCD, said optical reduction systems have the undesirable affect of effectively decreasing the resolution of the CCD.
The optical system, itself, degrades image resolution to some degree, but, the actual reduction process is the factor which most severely degrades the effective resolution of the image formed by a CCD. For example, a typical two dimensional CCD array is one centimeter square and includes therein approximately 256,000 photosensor units, generally referred to as pixels. To translate this into photographic terminology, the equivalent resolution would be about 50 lines/mm for the one centimeter square CCD array. When an image or other pattern of information occupying an area of 35.times.35 millimeters is projected onto this one centimeter square charge coupled device, the effective resolution of the 35 millimeter square image is reduced to approximately 15 lines/mm. For the sake of comparison, medium resolution photographic film is generally capable of resolving approximately 120 lines/mm.
Efforts to improve resolution using single crystal integrated circuits, such as CCDs, have encountered at least two significant problems. The first problem is that integrated circuit chips formed on single crystal silicon wafers must be as small as possible to provide acceptable yields and to meet the requirements imposed by the economies of manufacturing. The second problem, which is intimately related to the first, is that in order to increase the packing density of photosensor elements in the small available chip area, finer and finer photolithography must be used, with resulting increases in the cost of manufacture. For these reasons, among others, high resolution electronic photography (high resolution being defined relative to chemical photographic capabilities) utilizing present CCD technology is not economically feasible. A direct analogy would be that employing conventional CCD technology in a camera is akin to taking photographs on high grain (50 lines/mm) photographic film, utilizing a format which provides negatives 1 centimeter square. It is simply not possible to obtain good quality enlargements from such a combination.
With improvements in lithographic techniques, it is anticipated that one centimeter square CCDs may ultimately be fabricated to include 1.4 million pixels therein. This translates to a resolution of approximately 120 lines/mm on the one centimeter square device and a corresponding effective resolution of 34 lines/mm for a 35 millimeter square pattern of information projected thereonto. The only way the resolution of the CCD could be further increased is by increasing either the density of pixels in the CCD or the size of the device itself. Both approaches present significant problems. On one hand, the diffraction limit of light will ultimately impose limits on any photolithographic process utilized to pattern CCDs, although constraints of practicality and cost will generally intervene first to set an economic limit on pixel density. On the other hand, processing constraints will limit the size of crystalline CCDs which can be manufactured. Single crystal wafers cannot generally be economically manufactured in sizes exceeding perhaps six to eight inches in diameter. Furthermore, processing steps introduce defects and hence, severe yield-related restrictions into such devices.
Increasing the size of a crystalline device, such as a CCD, especially while maintaining strict limits on the size of the photolithographic features thereof, imposes a great burden of cost insofar as the likelihood of creating defects on a device exponentially increases along with a dramatic decrease in the yield. The result is that the cost of the finished product increases exponentially with increasing device size. It should thus be appreciated that even utilizing the most optimistically projected pixel densities and single crystalline CCD sizes, electronic cameras capable of providing high resolution photographs of a practical size cannot be economically manufactured utilizing such technology.